Geotechnical News• December 2019
43
THE GROUT LINE
above the contact between rock and jet
grouting, in order to treat this transi-
tion zone with higher care.
In correspondence of the structures
founded directly on the bedrock, the
holes were drilled using as guides
some plastic tubes previously embed-
ded in the cast concrete.
The upper obturator bag was placed
about 1 m (3 ft) under the founda-
tion level, in order to obtain a sealed
contact between rock and concrete.
In this area a third alignment of pipes
was executed, inclined towards the
reservoir, in order to guarantee a better
treatment from the side of the hydrau-
lic head.
Final controls on the impervious
curtain
After the completion of the jet grout-
ing and rock grouting treatments, on
each site a final investigation cam-
paign was carried out. Lugeon and
Lefranc tests (rock and jet grouted
soil) were carried out on inclined
cored holes along the impervious
curtain and at different depths.
The tests always showed the fulfill-
ing of the prescribed permeability
coefficient, as plotted in the following
charts in fig. 13: the diagram on the
left shows the permeability values
before and after the jet grouting treat-
ment in the soil, whereas on the right
the permeability in Lugeon Units (UL)
before and after the rock grouting.
Permeability values after the treatment
are plotted with full symbols
Summary and Conclusions
The cascade HPP of Iskar River, in
Bulgaria, is composed of nine plants.
For each of them, the design required
the execution of a cofferdam for
protecting the power house excava-
tion site from the river flow, and of
an impervious curtain, along the earth
dam and the structures section, against
the water seepage in the ground,
composed of rock basement underly-
ing alluvial coarse soil. Until today,
five plants are completed and regularly
operating.
Jet grouting columns reinforced with
steel pipes were executed along the
provisional embankment in order to
make it waterproofed and to work as a
gravity structure able to withstand the
hydraulic effort of the water even in
case of floods. This solution allowed
to avoid any seasonal interruption dur-
ing the works in the riverbed, gaining
between 4 and 6 month in the execu-
tion schedule of each plant.
The impervious curtains have been
executed by combining the use of
jet grouting for the treatment of the
coarse soil layers, and the MPSP sys-
tem for the grouting of the underlying
bedrock. The adoption of the MPSP
allowed to save time during the treat-
ment of crushed rock and to obtain a
highly homogenous watertight treat-
ment, particularly sealing some critical
zones such as: the contact between
the rock and the jet grouted soil; the
upper part of the rock mass, weakened
from the excavations (usually made by
blasting); the contact between the rock
and the concrete foundation structures
of the power house and the weir. Final
permeability controls on site into the
curtains confirmed the good outcome
of the treatments.
Acknowledgments
A special acknowledgment to Pla-
men Dilkov, construction manager of
Vez Svoghe OOD that owns and has
built the plant, who directed the works
with the support of Patrick Pauletto,
to Achille Balossi Restelli and Elena
Rovetto who have contributed to the
development and the design of part
of the project, to Injectosond S.r.l.,
specialized company executor of the
treatments.
References
Balossi Restelli A., Rovetto E. and
Fava A. R., 1993. La tratta Dine-
gro – Principe della Metropolitana
di Genova. Atti del XVIII Con-
vegno Nazionale di Geotecnica,
Rimini (Italy), pp. 51-60.
Balossi Restelli A., Tornaghi R., Pet-
tinaroli A: and Rovetto E., 2003.
Reconstruction of La Fenice
Theatre in Venice – Foundations
problems. 13th European Con-
ference on Soil Mechanics and
Geotechnical Engineering, Prague,
Vol.2, pp. 29-34
Bruce D. A., and Gallavresi F., 1988.
The MPSP system: a new method
of grouting difficult rock forma-
tions. ASCE Conference, Nash-
ville, TN.
Manassero V., 1993. Different tech-
niques for soil improvement and
Figure 12: Cross sections of the treatment for the
impervious curtain along the earth dam.
Figure 13: Final controls on the impervious curtain:
Lefranc test in soil (on left), Lugeon test in rock (on right)
white and black dots. before and after the grouting.
